The present invention is related to increasing the signal to noise ratio of distributed acoustic sensing by spatial averaging.
Distributed acoustic sensing (DAS), also referred to as phase or coherent optical time domain reflectometery (OTDR), is the use of an optical fiber to sense acoustic vibrations at any spatial point along the optical fiber's length. As schematically shown in FIGS. 1A-1D, in DAS a laser pulse is launched into the optical fiber and, similar to conventional OTDR, an OTDR “trace” is measured, i.e., Rayleigh backscattered light is measured at every spatial point along the optical fiber's length using a time of flight analysis. FIG. 1A shows a schematic of distributed acoustic sensing (DAS), FIG. 1B shows one optical fiber, FIG. 1C shows N optical fibers, and FIG. 1D shows a multicore optical fiber with N cores. In contrast to conventional OTDR, in DAS the phase of the Rayleigh backscattered light is also measured. If an acoustic vibration makes physical contact with the optical fiber at a spatial point along its length, due to the photoelastic effect, the phase of the Rayleigh backscattered light at that point will be directly proportional to the acoustic vibration's amplitude. By repetitively launching laser pulses into the optical fiber, the acoustic vibration's signal, comprising its amplitude and frequency, can be detected.
As optical fibers can be embedded in various structures, DAS is attractive for applications that require the sensing of acoustic vibrations over long distances with spatial accuracy including, monitoring bridges, tunnels, railroads, buildings, oil wells, borders, among others.
One problem of DAS is increasing the signal to noise ratio (SNR) of the acoustic vibration's detected signal. The noise of DAS has many sources including, laser phase noise, laser intensity noise, amplified spontaneous emission noise originating from the use of optical amplifiers, such as, Erbium doped fiber amplifiers, etc. If SNR is increased, effectively, the length of optical fiber over which an acoustic vibration can be detected will be increased. Additionally, if SNR is increased, at a given optical fiber length, the sensitivity of the detection of an acoustic vibration, i.e., the smallest detectable acoustic vibration, will be increased.
In conventional OTDR, SNR can be increased by temporal averaging—In temporal averaging, laser pulses are repetitively launched into the optical fiber. An OTDR trace is measured for each laser pulse. The OTDR traces are then digitally added. It is assumed that successive OTDR traces are identical—they do not change over time. The noise is assumed to be statistically random while the signal is not. Therefore, by adding successive OTDR traces, noise will add “slower” than the signal, and SNR will increase. However, in contrast to conventional OTDR, in DAS successive OTDR traces are not identical as they correspond to a different phase of the acoustic vibration. As a result, temporally averaging is not possible.
Another solution where SNR can be increased is by increasing the power of the laser pulse that is launched into the optical fiber; more light improves receiver performance. However, there is a limit to how much power can be launched into the optical fiber. Beyond this limit the light will experience nonlinear interactions with the optical fiber that can detrimentally affect the signal of the acoustic vibration. Additionally, beyond a certain power limit certain noise sources, such as, detector shot noise, will increase more than the signal of the acoustic vibration.
Yet another solution is coding. In “coding,” similar to temporal averaging, laser pulses are repetitively launched into the optical fiber. An OTDR trace is measured for each laser pulse. However, in coding the light pulses launched into the optical fiber are “coded”, i.e., they are launched at discretized time intervals. A unique OTDR trace is measured for each coded laser pulse. The measured OTDR traces are digitally processed to recover the desired OTDR trace. Using coding, the increase of SNR is greater than temporal averaging In coding, it is assumed that for each launched laser pulse, the desired OTDR traces are identical—they do not change over time. However, in contrast to conventional OTDR, in DAS successive OTDR traces are not identical as they correspond to a different phase of the acoustic vibration. As a result, coding is not possible.
Therefore, unlike conventional OTDR, the SNR of DAS cannot be increased by temporal averaging, increasing the power of the launched laser pulse, or with coding. Therefore, here is great need of a method to increase the SNR of DAS. By increasing the SNR of DAS, the length of optical fiber over which an acoustic vibration can be detected will be increased and at a given optical fiber length the sensitivity of the detection of an acoustic vibration, i.e., the smallest detectable acoustic vibration, will be increased.
Others have attempted to increase the SNR of DAS by Raman amplification. In Raman amplification, a laser pulse is launched into an optical fiber and, similar to conventional OTDR, an OTDR “trace” is measured, i.e., Rayleigh backscattered light is measured of at every spatial point along the optical fiber's length using a time of flight analysis. However, the OTDR trace is amplified using Raman amplification, i.e., a Raman pump light beam is also launched into the optical fiber. Raman amplification increases the signal of the acoustic vibration and in turn increases the SNR of DAS.
Unlike conventional OTDR, the SNR of DAS cannot be increased by temporal averaging, increasing the power of the launched laser pulse, or with coding. Therefore, there is great need of a method to increase the SNR of DAS.